Precise determination of the energy level of various solar materials for high-power devices

An electronic mismatch in the pairs of materials used to make organic solar cells can improve cell operation, KAUST researchers have shown. The surprising finding – published in Advanced Materials and was created following a comprehensive reanalysis of cutting-edge organic solar materials – should allow researchers to predict new organic formulations with strong solar cell performance.

The light-trapping layer of organic solar cells typically consists of a two-component mixture called a donor-acceptor mixture. In sunlight, the incident energy can excite electrons to hop between different energy levels in the mixture, suggesting that energy level alignment between the donor and acceptor components should be key to device performance. However, different research groups have used different methods of measuring energy levels and have produced conflicting results regarding the effect of energy level alignment.

“We wanted to understand the origin of this conflicting understanding,” says Anirudh Sharma, a postdoc in Derya Baran’s lab who led the work. “This motivated us to investigate the energy levels of organic solar cell materials using different methods in order to establish meaningful material-property relationships,” he says.

The team determined the energy levels of 33 organic solar materials using four different techniques. “Our results show that the energy levels of organic semiconductors, measured with photoelectron and low-energy inverse photoelectron spectroscopy, correlate closely with the properties of solar cells,” says Jules Bertrandie, Ph.D. Student in Baran’s group. He notes that cyclic voltammetry, in contrast, gave misleading results related to the energy levels of solar cells.

After determining the best methods for measurement, the team analyzed the alignment of energy levels in 12 donor-acceptor organic solar cell mixtures. The results showed – contrary to current belief – that mixtures with little to no difference in an energy level metric known as the ionization energy performed poorly. The highest performing organic solar cells had a significant difference in ionization energy between the donor and acceptor components in the mixture.

“We weren’t surprised that our proposal for a new way of measuring energy levels would be more reliable, but we were surprised that our hypothesis revealed that what is commonly believed in the literature is actually not true,” says Baran . “This method has enormous potential to reveal new donor-acceptor pair systems that could work well in organic solar cells and would also reveal which pairs are not working,” she adds.

The significance of the finding goes beyond solar research, Sharma notes. “The implications of this study are relevant to a larger field of organic electronics in general,” he says.